Directional couplers



Jan; I4, 1958 P. J. sFERRAzzA DIRECTIONAL COUPLERS Filed March 18., 1954 Q Q 0 @@@ouw C o QGGQD, W wPmll BY ,Jn

United States atent DIRECTIONAL COUPLERS Peter J. Sferrazza, Wantagh, N. Y., assignor to Sperry Rand Corporation, a corporation of Delaware Appiication March 18, 1954, Serial No. 417,072

4 Claims. (Cl. S33- 10) This invention relates to improvements in directional couplers, and more particularly to directional couplers of the waveguide type, in which a main waveguide or primary waveguide is coupled to a branch or auxiliary waveguide by means of one or more arrays of coupling elements such as apertures in a common wall that separates the two waveguides.

Directional couplers are used in the microwave radio art for various purposes such as power monitoring, measurements, isolation, and other applications which require that power flowing in one direction in the main guide will induce a flow of power only in a corresponding direction in the auxiliary guide. One of the measures of the quality of a directional coupler is its ability to discriminate between power owing in one direction in the main guide and power owing in the other direction, i. e. the ratio of the power induced in the auxiliary guide in the desired direction to that induced in the opposite direction, by power owing in one direction in the main guide. This ratio is called the directivity, and is usually expressed, in decibels.

Another characteristic that species the performance-V of a directional coupler is its attenuation. This is the ratio of the power induced in the desired direction in the auxiliary guide to the power in the main Aguide that induces it. Often it is desirable that the directivity be relatively high, and the attenuation be nearly constant, throughout a substantial range of operating frequencies. The frequency range over which the directivity and attenuation remain within specified tolerances may be regarded as the bandwidth of the coupler.

' The principal object of this invention is to provide an improvement in directional couplers whereby there is attained a directivity that is substantially greater than that of similar prior art directional couplers.

Another object is to achieve the above-mentioned improvement in directivity without any corresponding increase in the complexity of design or construction of the directional coupler, and without any sacrifice in bandwith or constancy of attenuation.

The invention will be described with reference to the accompanying drawing, wherein:

Fig. 1 is a view in elevation, partly in section, of a directional coupler constituting a presently preferred embodiment of the invention, and

Fig. 2 represents a section along the plane II-II of the structure of Fig. 1.

Directional couplers of the type to which the present invention relates generally comprise two waveguides that are joined throughout a portion of their lengths to provide a common wall, as shown in Fig. l, and some kind of coupling means such as a plurality of apertures in the common wall for transferring wave energy from one guide to the other. In the structure of Fig. l the main waveguide 1 and the auxiliary waveguide 3 are both of rectangular cross section, with their respective adjacent broad sides joined to form a common wall portion 5. The main guide 1 is provided at its ends with fittings 7 and 9 for connection with external apparatus such as a source and a load, respectively, and one end of the auxiliary guide 3 has a similar tting 11 for connection to y 2,820,203 Patented Jan. 14, 1958 ICC other apparatus, for example a power measuring instrument. The other end of the guide 3 is terminated with a resistive load to minimize reflections therefrom.

One of the simplest arrangements for coupling the waveguides in a coupler like that of Fig. l consists of two identical holes in the common wall, spaced apart along the longitudinal axis by a distance equal to one quarter the guide wavelength at a specified operating frequency. When power is flowing in the main waveguide, a small part of it couples through each of the holes, exciting waves in the auxiliary guide that propagate in both directions from each hole. At the particular frequency where the spacing between the holes is one-quarter wavelength, the waves in the auxiliary guide traveling in the direction opposite to the power flow in the main guide cancel each other, while those traveling in the same direction as those in the main guide reinforce each other.

The directivity of the two-hole coupler deteriorates rapidly as the operating frequency departs from that for which the coupler is designed; i. e. the bandwidth of such a coupler is narrow. Considerable improvements in bandwidth may be obtained by modifying the design of the coupling arrangement to use a larger number of holes, with the hole sizes distributed in such a way as to form the equivalent of an array of overlapping two-hole couplers, spaced at quarter-wavelength intervals along the axis. The simplest array of this sort consists of three coupling elements or holes spaced at quarter-wave intervals, with the center one designed to couple twice as much voltage into the auxiliary guide as each of the end couplingelements. This gives the effect of two identical two-hole couplers, spaced apart one-quarter wavelength.

"Similarly a four-hole coupler can be designed as an array of two overlapping three-hole arrays, and so on. Distributions of this kind are known as binominal arrays, because the voltages required to be coupled through successive holes in the series are proportional to the coefficients of a binomial expansion whose order is one less than the number of holes. Directional couplers using binomial arrays are described in U. S. Patent 2,562,281.

- The directivity of a binomial. 'array remains reasonably high throughout a frequency band whose width depends upon the number of coupling elements. However, the directivity decreases steadily as the operating frequency departs from the design frequency.

A more eicient array, from the standpoint of the number of holes required to achieve a specified minimum directivity over a given frequency band, is the Tschebysche type, so named because the coupling amplitudes of the successive holes in the series are proportioned so that the amplitude of the backward wave induced in the auxiliary waveguide oscillates as a function of frequency in a manner corresponding to the variations of a Tschebyscheff polynomial whose order is one less than the number of coupling holes. Tschebyscheff polynomials are functions whose values oscillate between limits of |1 and l as the value of the independent variable is varied from -1 to +1. The number of oscillations over this region depends upon the order of the polynomial.

By expressing the amplitude of the backward wave coupled into the auxiliary guide in the form of a Tschebyscheif polynomial of the order corresponding to the number of coupling holes, less one, and choosing units to make the ranges from -1 to +1 correspond to the desired bandwidth and the permissible variation `in directivity, respectively, the required coupling amplitudes may be solved for. The use of the basic Tschebyscheif distribution in directional couplers is described inthe Proceedings of the I. R. E. volume 39, page 291 (March 1951) by F. Bolinder.

In cases where relatively low attenuation is required together with high directivity, itis found that the basic Tschebyscheff array becomes impractical because it must be too long, and the ratio of the largest holes to the smallest is too great. This diiculty can be alleviated by overlapping several basic arrays, thereby making the overall coupling region shorter and making the requiredhole sizes more nearly uniform. Such overlappingdecreases the attenuation in accordance with the numbenof basic arrays used without adver'selyaiecting the directivity. For still lower attenuation, two or more arrays may be placed side by side in the common wall between the guides, each array being overlapped. This arrangement provides vthe desired lower attenuation but retains substantially the same directivity 'as the basic array. A double row triply overlappedl Tschebysdiet coupler of this design is described in -the"M arch-`April 19,52 issue of the Hewlett-Packard Journal, published by the Hewlett- Packard Company, Palo Alto, California.'

' According to the present invention, the directivity of multiple-row couplers is materially improved withoutv any increase' in attenuation' orcomplexity' of structure, and with only a small Vincrease in length,` by spacing the rows of coupling elements apart longitudinally; Referring to Fig. 2, the coupling arrangement' 'of the illustrated embodiment of the invention comprises two rows 13 and of circular holes arrangedl side byside in the wall 5V and displaced longitudinally with respect to each other by one quarter the guide wavelength at lthe center of the frequency'band over which the coupler is to be used. Each of the rows 143 and 1S is designed as a triply overlappedTschebyschei array. The center line of each 'row is locatedabout one quarter of the guide width from the respective edge of the 'wall 5`to minimize variations,

the individual holes.

` In'a typical coupler, designed for operation over the frequencyfrange designated in theart as lX b'and,'th`e` pertinent` dimensions' are as follows: i l

waveguides, lf 'X1/2 "X.O5,0', wall Coupling hole diameters in each row, starting withrthe center hole and proceeding either way,

Lateral spacingof each row from, the

'the common wall, .225"

Longitudinal spacing between holes, .380"r Longitudinal Vdisplacement between the two rows, .380W` Theaverage directivity of the described coupler, over the operating frequency band, is about 6 db. higher thany that of a similar coupler wherein the rows 'of'coupling elements are not'displaced longitudinally.I This improvement in performance is effected without any increasein the difiiculty of designing or Vfabricating the device.

Although theinvention has been'described as embodied inav specific designfof directional coupler, it may heemployed in any multiple-row type of coupling arraywith similar advantage. i

'Since many changes could be made in the above .constructionvand many apparently widely"different embodiments of this invention could be made without departing from the scope thereoit is intended that all matter contained in the above description or shown in the ,accompanying drawings shall be interpreted as ,illustrativefand not in a limiting sense.

What is claimed is;

l.` A ,directional coupler comprising a main waveguide and an auxiliary waveguide, said waveguides being dparallel and adjacent throughout a portion' of their lengths as a function of frequenemof the couplings providedbyf' respective edge of fs, Provide a .Cowles region. @plurality 0f angestellt identical rows of coupling elements in said region, each of said rows extending along a respective line parallel to the longitudinal axes of said guides, the elements of each of said rows being spaced apart longitudinally at equal intervals of substantially one quarter guide wavelength at the center of the frequency band throughout which the device is to operate, whereby each row of elements constitutes a directional coupling array, said rows being displaced longitudinally with vrespect to each other by a distance equal to saidlongitudinal intervals between coupling elements.

2. A directional coupler comprising two rectangular waveguides having a common broad wall, a first directional coupling array comprising a row of apertures in said `wall disposed at equal intervals of substantially one quarter the guide wavelength at the center of the frequency band throughout which the device is to operate along a rst line parallel to the longitudinal axis of said wall, said apertures having relative proportionssuch as to provide an overlapped Tschebyscheit array, and a second coupling array identical with said first array but disposed along a second line on the opposite side of said axis from said first line, each of said lines being spaced laterally from therespective edge of said broad wall by a distancey ofpsubstantially one-quarter the width of said wall, said arrays being displaced longitudinally with respect to eachother by a distance equal to said longitudinal interval between apertures.V

` 3. A directional coupler comprising two waveguides having a common wall, a first directional array of coupling elements, said array being substantially longer than one wavelength at the-lowest frequency of operation and extending along a line parallel to but laterally spaced froml the longitudinalaxis of symmetry of said common wall, and a second directional array of coupling elementsy substantially identical to said first coupling array but disposed along a-y second line laterally spaced from said longitudinal axison the other side from said first line, said second arraybeing longitudinally displaced with respect to said .rst array by a distance less than the length of one of said arrays, the longitudinal displacement between said couplingarrays being substantially one-quarterv the guide wavelength at the middle of the frequency band throughoutwhichthe device is to operate.

4. A directional coupler comprising a main waveguide and an auxiliaryY waveguide, said waveguides being joined throughout aportion of their lengths to provide a common wall, a pluralitypf substantially identical rows of apertures in said common wall, each of said rows extendingalorlga respectiverline parallel to thelongitudinal axis offsaidwall, saidilines.being ,spaced apart laterally of said wall, the apertures in-each of said rows beingspaced apart longitudinally yatequal intervals of substantially onequarter wavelength at the center of the band of operating frequencies, said rows beingdisplaced longitudinally with respect toeach other by a distance substantially less thanA thei lengths ofsaid rows, the longitudinal displacement between said rows is substantially one-quarter the guide wavelength at the center of` the 4frequency band throughout which the device is to operate.

References Cited in thele of this patent UNITED STATES PATENTS 2,512,191 i Wolf June V20, 1950 2,585,173 Riblet Feb. 12; 1952 2,701,341 Bowen Feb. 1, 1955 2,720,629 Edson..."V Oct. 1l, 1955y FOREIGN PATENTS 686,062 Great- Britain v Jan. 14,1953,

OTHER REFERENCES Publication 1, Barnett et al., A Precision Directional Coupler Using Multihole Coupling, Hewlett-Packard Journal, viol.l 43,'vl\los.- 7-,8 llt/[arch-April4 l19,52. 

